1. Field of the Invention
The present invention generally relates to channels for exchanging messages between central processing complexes and message processors, including input/output (I/O) devices and systems and other central processing complexes, and more particularly to high performance buffering which facilitates very low latency communications between elements of very high speed data processing systems. The invention allows true peer-to-peer communications between central processing complexes and implements a recovery procedure of individual operations in a multiple operation environment.
2. Description of the Prior Art
In a conventional data processing system, a central processing complex (CPC) having a channel path is connected to a control unit to which are attached one or more strings of direct access storage devices (DASDs), such as disk drives. The control unit includes a buffer for temporarily storing data sent by the central processing complex to be written to one or more of the DASDs. The central processing complex and the control unit operate in a master/slave relationship. The sending of data between the central processing complex channel and the control unit requires a number of handshaking messages to be sent between the two before data can start to be transmitted. The time it takes to do this handshaking slows down the performance of the data transfer significantly.
Various techniques have been employed to gain a marginal improvement in performance. One such technique involves a "shadow write" operation wherein the data transmitted by the central processing complex is buffered in the control unit but not written to DASD until later. Nevertheless, the central processing complex is notified by the control unit that the write operation has taken place, thereby eliminating the time delays normally associated in the electromechanical write operations.
Multiprocessor (MP) systems have been developed to increase throughput by performing in parallel those operations which can run concurrently on separate processors. Such high performance, MP data processing systems are characterized by a plurality of central processor units (CPUs) which operate independently and in parallel, but occasionally communicate with one another or with a main storage (MS) when data needs to be exchanged. In the type of MP system known as a tightly coupled multiprocessor system in which each of the CPUs have their own cache memory, there exist coherence problems at various levels of the system. A number of solutions to this problem are known in the art. One approach involves a cross-interrogate (XI)technique to insure that all CPUs access only the most current data.
Recently, there have been developed massive electronic storage devices which are replacing the slower, electromechanical DASDs used in older systems. These electronic storage devices, while representing a significant increase in the speed of MP systems, do not address the prior problem associated with the handshaking protocol of prior systems. Added to that is the cross-interrogate (XI) process that generally characterize memory management in a tightly coupled MP system.
There are also known data processing systems in which a plurality of central processing complexes (CPCs) are interconnected via a communication link. The CPCs run independently but must communicate with one another to transfer and/or process data. The CPCs may be large main frame computers which communicate via some message processor, or they may be a plurality of individual work stations communicating over a local area network (LAN) or wide area network (WAN) which might typically include a server. In either case, communication is typically handled in a master/slave relationship, even between large main frame computers. The master/slave designation changes depending on the flow of data, but the type of handshaking protocol described for I/O devices is typical resulting in delays in data transfer.
High speed data processing systems and elements are being developed wherein the communication process is, in many cases, the limiting factor in data throughput. The transmission capacities of the media, notably copper, has been a limiting factor in the performance of communications. New media, such as optical wave guides, e.g., fiber optic cables, have significantly higher transmission capabilities than was possible with previous media. There is considerable need for overall improvement in data communications and data throughput in high performance data processing systems.